In general, redox flow batteries are used for load leveling or for countermeasure to voltage sag (momentary drop in voltage). FIG. 3 shows an explanatory view showing an operating principle of a redox flow secondary battery. This battery has a cell 100 which is separated into a positive electrode cell 100A and a negative electrode cell 100B by a membrane 103 of an ion-exchange membrane. A positive electrode 104 and a negative electrode 105 are contained in the positive electrode cell 100A and the negative electrode cell 100B, respectively. A positive electrode tank 101 for feeding and discharging positive electrode electrolyte to and from the positive electrode cell 100A is connected to the positive electrode cell 100A through conduit pipes 106, 107. Similarly, a negative electrode tank 102 for feeding and discharging negative electrode electrolyte to and from the negative electrode cell 100B is connected to the negative electrode cell 100B through conduit pipes 109, 110. Aqueous solution containing ions that change in valence, such as vanadium ion, is used for the respective electrolytes and is circulated by using pumps 108, 111, to charge or discharge with an ionic valence change reaction on the positive and negative electrodes 104, 105. For example, when the electrolyte containing the vanadium ions is used, the following reactions occur in the cell during the charge or discharge of electricity:
Positive electrode: V4+→V5++e− (Charge) V4+←V5++e− (Discharge)
Negative electrode: V3++e−→V2+ (Charge) V3++e−←V2++ (Discharge)
FIG. 4 is a schematic block diagram of a cell stack used for the battery. A structure comprising a plurality of sub-stacks 201 stacked in layers, each comprising a plurality of cells stacked in layers, what is called a cell stack 200, is used for battery described above. Each cell has the positive electrode 104 made of carbon felt and the negative electrode 105 made of carbon felt which are arranged at both sides of the membrane 103. Cell frames 210 are arranged at the outside of the positive electrode 104 and at the outside of the negative electrode 105, respectively. Each cell frame 210 comprises a bipolar plate 211 made of a plastic carbon and a frame 212 surrounding the bipolar plate.
The frame 212 has a plurality of holes, which are called manifolds, formed therein. Each cell frame has e.g. eight manifolds in total, four in a lower side thereof and four in an upper side thereof. Two of the four manifolds in the lower side of the cell frame are used for supplying positive electrode electrolyte, and the remaining two are used for supplying negative electrode electrolyte. Two of the four manifolds in the upper side of the cell frame are used for discharging the positive electrode electrolyte, and the remaining two are used for discharging the negative electrode electrolyte. The manifolds are formed into flow channels for the electrolytes to pass through by stacking a number of cells in layers and in turn are connected to circuit pipes 106, 107, 109, 110 in FIG. 3. The electrolytes are supplied and discharged in each of the sub-stacks 201. As shown in FIG. 2, electrolyte supplying pipes 220, 221 for supplying the positive electrode electrolyte and the negative electrode electrolyte and electrolyte discharging pipes 222, 223 for discharging the positive electrode electrolyte and the negative electrode electrolyte are connected to each of the sub-stacks 201.
The sub-stacks 201 are electrically interconnected through conductive plates 224, such as copper plates, interposed between adjacent sub-stacks. Each sub-stack 201 has electrical terminals (not shown) provided on a side thereof different from the sides on which the electrolyte supplying pipes 220, 221 and the electrolyte discharging pipes 222, 223 are provided. The entirety of the cell stack 200 is usually connected to a DC/AC converter 225 through the electrical terminals.
For the load leveling, this redox flow battery is commonly operated to stop charging and discharging based on an upper limit and a lower limit of a predetermined distribution voltage. The stop of charging and the stop of discharging are both determined with reference to the distribution voltage (a voltage of the cell when the battery is in operation). Also, the charging state (charging rate) of the electrolyte in the cell is commonly grasped with reference to a circuit voltage (a voltage of the cell when the battery is in non-operation).
The conventional redox flow battery has the following problems, however.
(1) When the stop of charging and discharging is determined with reference to the distribution voltage, variations in charging rate of the cell may be caused.
Operating conditions of the battery vary depending on changes in battery resistance caused by degradation of the battery, variation in the environment, such as temperature variation, and the like. For example, generally speaking, the higher the temperature is, the more effectively the battery can charge and discharge. When the stop of charging and the stop of discharging are determined with reference to the distribution voltage, the variations in operating condition may cause variations in charging rate of the cell, i.e., variations in output capacity of the cell (kWh), at the stop of charging and discharging.
(2) It is difficult for the conventional redox flow battery to constantly grasp the charging rate of the cell.
Measurement of the circuit voltage requires the halt of the operation of the battery. Consequently, trying to grasp the charging state (charging rate) of the cell constantly with reference to the circuit voltage as usual requires the continuous halt of the operation of the battery. Hence, this is not a realistic way. As is known in recent years, the redox flow battery is often combined with a wind power generation system or a solar power generation system, to provide a stabilized output capacity. In this combination, the charging rate of the cell cannot be changed properly without grasping the charging rate of the electrolytic and, as a result, the battery may fail to charge and discharge electricity sufficiently. Accordingly, grasping the charging rate of the cell constantly is being desired.
It is a primary object of the invention to provide an operating method of a redox flow battery capable of grasping a charging rate of the battery further reliably to stabilize an output capacity of the battery, and provide a cell stack optimum for this operating method.